The underlying hypothesis that is being testes is "that there is a specific set of irradiation conditions for laser light that most efficiently and effectively interacts with dental hard tissues". Treatment of dental enamel by carbon dioxide (CO2) laser light has been shown to markedly inhibit subsequent acid-induced demineralization. The efficient conversion of light energy to heat when a laser beam interacts with dental hard tissues is dependent on several laser parameters, including wavelength, pulse width and fluence. The potential is excellent for the clinical use of specific wavelength lasers for protection against subsequent caries attack and for the treatment of early lesions. Before the clinical application can be realized the scientific basis for the choice of the specific conditions must be established and safety issues must be addressed. The purpose of the studies proposed here is to establish that basis. The objectives will be achieved by the following specific aims; (1) To determine the absorption coefficients for dental hard tissues at selected wavelengths (9.3.9.6. 10.3 and 10.6 mum) of CO2 laser radiation. This will be achieved by using a combination of detailed measurements of surface temperatures and extensive thermal simulations of heat conductance and phase transformations. (2) To study the thermal behavior of enamel, dentin, and synthetic apatite using selected conditions of CO2 laser irradiation. This aim has several components. (a) Study of the heart wave propagation and comparison with numerical simulations. (b) Study of single and multiple-pulse irradiation effects in order (i) to understand past multiple-pulse experiments, (ii) to evaluate their effectiveness to cause recrystallization and/or reactivity modification over an extended depth, and (iii) to evaluate the thermal assault of the interior of the tooth, particularly athe pulp region. This segment will lead to future more extensive safety studies. (30 To carry out in vitro experiments to determine CO2 laser wavelength and irradiation conditions that (a) reduce the solubility of dental mineral, and (b) inhibit the progression of caries-like lesions in enamel and tooth roots. Surface changes resulting from CO2 laser treatment will be assessed by scanning electron microscopy, x-ray diffraction, infrared spectroscopy, chemical analysis, and by detailed dissolution studies using dual constant composition methodology. The inhibitory effect of treatment by a range of CO2 laser irradiation conditions on progression of artificial caries-like lesions in (a) sound dental enamel, (b) sound tooth roots, an (c) initial caries-like lesions in (i) smooth surface enamel; (ii) occlusal pits and fissures of enamel; (iii) tooth root surfaces will be assessed by an in vitro pH cycling demineralization/remineralization model. (40 To determine laser wavelengths and conditions that will form the basis of future clinical studies. It is anticipated that the present studies will provide a range of specific laser conditions that can subsequently be examined with regard to safety and clinical efficacy. If successful, this would provide improved methods for treating pit and fissure caries, root caries nd early smooth surface enamel caries.